The polybenzoxazole is a super-engineering plastic that is very excellent in heat resistance, mechanical strength, dimensional stability, insulating properties and the like, and has been extensively used in the applications of electric and electronic equipments such as printed wiring boards and circuit boards.
The polybenzoxazole is obtained by condensation of an aromatic dicarboxylic acid compound and a bis(o-aminophenol)-based compound. However, the reactant in the form of a carboxylic acid has a low reactivity with phenol or amine, and therefore various studies on a polymerization method therefor have been conventionally made. As to the polymerization method, for example, Non-Patent Document 1 discloses a method of subjecting a bis(o-aminophenol)-based compound and a dicarboxylic acid to condensation reaction in the presence of a polyphosphoric acid. In this method, the polyphosphoric acid acts as both a condensing agent and a solvent. Although the method can be readily performed in a laboratory scale, a large amount of phosphoric acid produced as a by-product will pose a significant problem concerning treatment thereof when practiced in an industrial scale.
Also, in the reaction between a dicarboxylic acid chloride as an activated form of a carboxylic acid and the bis(o-aminophenol)-based compound, there is only a less difference in reactivity between an amino group and a phenolic OH group, so that there tends to arise such a problem that the resulting polymer is insolubilized owing to formation of a crosslinking structure therein. To solve the above problem, there is known the method in which the bis(o-aminophenol)-based compound is subjected to trimethylsilylation reaction and then to the polymerization reaction. According to the above method, it is possible to selectively form an amido bond and thereby obtain a high-molecular weight compound of polyhydroxyamide as a precursor of the polybenzoxazole. The precursor can be treated at a temperature as high as about 250° C. to obtain the polybenzoxazole. However, since the dicarboxylic acid chloride is used in the method, it is necessary to remove a halogen-containing salt from the resulting product, which results in need of too much labor for purification treatment thereof.
To solve the above problems, for example, Patent Document 1 discloses a method for producing polybenzoxazole which is characterized by reacting a bis(o-aminophenol)-based compound with an aromatic dialdehyde. According to the method, it is possible to produce the polybenzoxazole without need of subjecting the obtained product to any particular purification treatment. However, a precursor of the polybenzoxazole produced by the above method tends to have a poor solubility in solvents, and therefore it will be difficult to obtain a uniform solution of the precursor. As a result, in the above method, there tend to arise disadvantages such as deteriorated film-forming property of the obtained polybenzoxazole and poor strength and brittleness of a molded article produced therefrom.
To solve the above problems, for example, Patent Documents 2 and 3 have proposed a specific structure of the above polybenzoxazole precursor which inhibits the precursor from undergoing a ring closing reaction in order to improve a solubility of the polybenzoxazole precursor. However, such a specific structure of the polybenzoxazole precursor which is free from a ring closing reaction by itself tends to be unstable and therefore tends to be deteriorated in storage stability. In addition, the polybenzoxazole precursor tends to be still deteriorated in solubility in solvents owing to a rigid structure on a dialdehyde side thereof, so that there tend to arise disadvantages such as difficulty in forming a uniform solution of the precursor, deterioration in film-forming property of the obtained polybenzoxazole, and poor strength and brittleness of a molded article produced therefrom.